No, personal-digest-preferences are not used to select a digest algorithm for key signatures. The only way to use a different digest-algorithm than select by gpg is by using --cert-digest-algo. But take care, you can easily cut into your fingers when using such override options.

GnuPG 1.4 uses SHA-1 to create key signatures and that is unlikely to change because 1.4 is a fallback solution to process exist000ing data created with PGP 2 keys or without the MDC feature. You should never ever create or modify a current key using that tool. The next maintenance release will emphasize on this by printing a bold warning if you try.

Regarding the "expire" or "set-primary" trick: My comment on heise.de was wrong - I overlooked that these two commands try to keep most properties of an old signature; including the digest algorithm :-(. This will be changed with the next GnuPG release to forcefully switch to SHA-256 if SHA-1 or RMD160 was used.

How a digest algorithim is selected for a key signature

Key signatures, which are self-signatures to bind a subkey or user-id to a key or 3rd part certifications of a user id, are created by gpg with a digest algorithm selected this way:

1. If the option --cert-digest-algo is used, the given algorithm is used regardless on whether this makes sense or is possible by the underlying public key algorithms. A wrong specification may lead to all kind of errors. Thus this option should in almost all cases not be used.
2. If a DSA or ECDSA public key algorithj is used, the required digest algorithm is selected. For example a 1024 bit DSA key requires a SHA-1 signatures, a 2048 bit DSA key requires SHA-256. All ECC algorithms use a SHA-2 algorithm.
3. The default algorithm is used. This means the use of SHA-256 in the default compliance mode (explicit with option --gnupg or --compliance=gnupg) or with --compliance=de-vs. In the various OpenPGP compliance modes (--rfc2440, --rfc4880, --openpgp, etc), SHA-1 is used due to the requirements out of the RFC.

The above is valid for the current stable GnuPG versions (>= 2.1). For GnuPG 1.4 the same algorithm is used but for compatibility with other OpenPGP implementations the default digest algorithm (for RSA) is still SHA-1.

Oh, ah. Ok. I do not read c't no more since about 2005. They are busy people and lead into the right direction.

Thank you for the investigative context information.

(Yes, it is RSA-4096 here. (I do not really care as i cannot, not being a mathematician, and RSA-4096 was advocated in the FreeBSD manual already in the FreeBSD time frame 4.7-5.3.)